(1) Field of the Invention
This invention relates to a method of producing acrylic fibers and more particularly to a method of producing acrylic fibers of uniform quality at high productivity by using a particular spinneret.
(2) Description of the Prior Art
Since acrylic fibers have a bulky touch resembling wool and excellent physical properties and dyeability, they are widely used in the field of clothing, bedding, etc. Continuous efforts are made toward the improvement of Productivity of the fibers.
A representative technique for such improvement of productivity is to increase the number of spinning orifices of the spinneret. When the orifice density is increased (i.e. when the orifice intervals are made small), single filaments agglutinate with one another, or when the size of the spinneret is enlarged, it is difficult for the coagulation liquid to penetrate into the inner part of the spun fiber bundle and this makes the coagulation uneven. Therefore, as a means for solving the problem of uneven coagulation which accompanies an increase of the orifice number, the use of annular type spinnerets has been proposed.
On the other hand, attempts have been made to improve the quality of the fibers and the to bestow new properties on the fibers. For example, the dry-wet spinning process (in which the spinning solution is once extruded into an inert gas and then introduced into the coagulation bath) described in Japanese Patent Publication No. 26212/1965 is known to provide acrylic fibers of dense fiber structure having a smooth surface and highly lustrous appearance and possessing excellent physical properties.
We investigated the effect of the conventional annular type spinnert. It became apparent that the coagulation liquid supplied from the outer peripheral side of the spinneret was not sufficiently supplied to the inner part of the spun fiber bundle, and that when the fiber bundle is gathered at the outlet of the coagulation bath, the coagulation liquid once supplied to the inner part of the fiber bundle has no way of escape, so that it circulates within the fiber bundle and puts the fiber bundle into disorder.
In the dry-wet spinning process, when using a spinneret having a small number of spinning orifices (up to 1,000 orifices at most) as in the case of producing filament yarn, it is possible to obtain fibers of excellent quality and performance as mentioned above, fairly without problems in operation. But with the increase of the number of orifices, it was found that the dry-wet spinning process causes the following various essential defects:
(i) The coagulation becomes uneven between the inner part and outer part of the spun fiber bundle;
(ii) The liquid filaments extruded from the spinneret (which do not substantially coagulate during the travel through the inert gas) easily agglutinate with one another by the contact between the filaments, and once they agglutinate, they do not separate easily;
(iii) The thicker the spun fiber bundle, the more liable is the coagulation liquid surface to be agitated. This causes fluctuation in the yarn quality.
(iv) Owing to the descending flow of the coagulation liquid accompanied by the spun fiber bundle, the coagulation liquid flows toward the central part of the fiber bundle. But with the increase of the number of spinning orifices, it becomes difficult for the coagulation liquid to be supplied to the inner part of the fiber bundle. Therefore, the liquid surface of the inner part of the fiber bundle becomes low, and at the same time the liquid surface of the outer part rises to a higher level, and thus the difference in the gap (distance between the spinneret surface and the coagulation liquid surface) among spinneret orifices becomes remakable. Or the spinneret may be dipped in the coagulation liquid, and consequently defective yarns such as thick-and-thin yarn, agglutinated yarn, abnormal denier yarn, etc. may increase markedly.
Taking into account the above-mentioned points which became clear from various experiments, we carried on studies of fiber production, and as a result, we reached this invention.